Energy Expenditure and Fatigue Chapter 5 Energy Expenditure and Fatigue

Chapter 5 Overview Measuring energy expenditure Energy expenditure at rest and during exercise Fatigue and its causes

Measuring Energy Expenditure: Direct Calorimetry Substrate metabolism efficiency 40% of substrate energy  ATP 60% of substrate energy  heat Heat production increases with energy production Can be measured in a calorimeter Water flows through walls Body temperature increases water temperature

Figure 5.1

Measuring Energy Expenditure: Direct Calorimetry Pros Accurate over time Good for resting metabolic measurements Cons Expensive, slow Exercise equipment adds extra heat Sweat creates errors in measurements Not practical or accurate for exercise

Measuring Energy Expenditure: Indirect Calorimetry Estimates total body energy expenditure based on O2 used, CO2 produced Measures respiratory gas concentrations Only accurate for steady-state oxidative metabolism Older methods of analysis accurate but slow New methods faster but expensive

Measuring Energy Expenditure: O2 and CO2 Measurements VO2: volume of O2 consumed per minute Rate of O2 consumption Volume of inspired O2 − volume of expired O2 VCO2: volume of CO2 produced per minute Rate of CO2 production Volume of expired CO2 − volume of inspired CO2

Figure 5.2

Measuring Energy Expenditure: Haldane Transformation V̇ of inspired O2 may not = V̇ of expired CO2 V̇ of inspired N2 = V̇ of expired N2 Haldane transformation Allows V of inspired air (unknown) to be directly calculated from V of expired air (known) Based on constancy of N2 volumes VI = (VE x FEN2)/FIN2 VO2 = (VE) x {[1-(FEO2 + FECO2) x (0.265)] − (FEO2)}

Measuring Energy Expenditure: Respiratory Exchange Ratio O2 usage during metabolism depends on type of fuel being oxidized More carbon atoms in molecule = more O2 needed Glucose (C6H12O6) < palmitic acid (C16H32O2) Respiratory exchange ratio (RER) Ratio between rates of CO2 production, O2 usage RER = VCO2/VO2

Measuring Energy Expenditure: Respiratory Exchange Ratio RER for 1 molecule glucose = 1.0 6 O2 + C6H12O6  6 CO2 + 6 H2O + 32 ATP RER = VCO2/VO2 = 6 CO2/6 O2 = 1.0 RER for 1 molecule palmitic acid = 0.70 23 O2 + C16H32O2  16 CO2 + 16 H2O + 129 ATP RER = VCO2/VO2 = 16 CO2/23 O2 = 0.70 Predicts substrate use, kilocalories/O2 efficiency

Table 5.1

Measuring Energy Expenditure: Indirect Calorimetry Limitations CO2 production may not = CO2 exhalation RER inaccurate for protein oxidation RER near 1.0 may be inaccurate when lactate buildup  CO2 exhalation Gluconeogenesis produces RER <0.70

Measuring Energy Expenditure: Isotopic Measurements Isotope: element with atypical atomic weight Can be radioactive or nonradioactive Can be traced throughout body 13C, 2H (deuterium) common isotopes for studying energy metabolism Easy, accurate, low-risk study of CO2 production Ideal for long-term measurements (weeks)

Energy Expenditure at Rest and During Exercise Metabolic rate: rate of energy use by body Based on whole-body O2 consumption and corresponding caloric equivalent At rest, RER ~0.80, VO2 ~0.3 L/min At rest, metabolic rate ~2,000 kcal/day

Energy Expenditure at Rest: Basal Metabolic Rate Basal metabolic rate (BMR): rate of energy expenditure at rest In supine position Thermoneutral environment After 8 h sleep and 12 h fasting Minimum energy requirement for living Related to fat-free mass (kcal  kg FFM-1  min-1) Also affected by body surface area, age, stress, hormones, body temperature

Resting Metabolic Rate and Normal Daily Metabolic Activity Resting metabolic rate (RMR) Similar to BMR (within 5-10% of BMR) but easier Doesn’t require stringent standardized conditions 1,200 to 2,400 kcal/day Total daily metabolic activity Includes normal daily activities Normal range: 1,800 to 3,000 kcal/day Competitive athletes: up to 10,000 kcal/day

Energy Expenditure During Submaximal Aerobic Exercise Metabolic rate increases with exercise intensity Slow component of O2 uptake kinetics At high power outputs, VO2 continues to increase More type II (less efficient) fiber recruitment VO2 drift Upward drift observed even at low power outputs Possibly due to ventilatory, hormone changes?

Figure 5.3

Energy Expenditure During Maximal Aerobic Exercise VO2max (maximal O2 uptake) Point at which O2 consumption doesn’t  with further  in intensity Best single measurement of aerobic fitness Not best predictor of endurance performance Plateaus after 8 to 12 weeks of training Performance continues to improve More training allows athlete to compete at higher percentage of VO2max

Figure 5.4

Energy Expenditure During Maximal Aerobic Exercise VO2max expressed in L/min Easy standard units Suitable for non-weight-bearing activities VO2max normalized for body weight ml O2  kg-1  min-1 More accurate comparison for different body sizes Untrained young men: 44 to 50 versus untrained young women: 38 to 42 Sex difference due to women’s lower FFM and hemoglobin

Energy Expenditure During Maximal Anaerobic Exercise No activity 100% aerobic or anaerobic Estimates of anaerobic effort involve Excess postexercise O2 consumption Lactate threshold

Anaerobic Energy Expenditure: Postexercise O2 Consumption O2 demand > O2 consumed in early exercise Body incurs O2 deficit O2 required − O2 consumed Occurs when anaerobic pathways used for ATP production O2 consumed > O2 demand in early recovery Excess postexercise O2 consumption (EPOC) Replenishes ATP/PCr stores, converts lactate to glycogen, replenishes hemo/myoglobin, clears CO2

Figure 5.5

Anaerobic Energy Expenditure: Lactate Threshold Lactate threshold: point at which blood lactate accumulation  markedly Lactate production rate > lactate clearance rate Interaction of aerobic and anaerobic systems Good indicator of potential for endurance exercise Usually expressed as percentage of VO2max

Figure 5.6

Anaerobic Energy Expenditure: Lactate Threshold Lactate accumulation  fatigue Ability to exercise hard without accumulating lactate beneficial to athletic performance Higher lactate threshold = higher sustained exercise intensity = better endurance performance For two athletes with same VO2max, higher lactate threshold predicts better performance

Measuring Anaerobic Capacity No clear, V̇O2max-like method for measuring anaerobic capacity Imperfect but accepted methods Maximal accumulated O2 deficit Wingate anaerobic test Critical power test

Energy Expenditure During Exercise: Economy of Effort As athletes become more skilled, use less energy for given pace Independent of VO2max Body learns energy economy with practice Multifactorial phenomenon Economy  with distance of race Practice  better economy of movement (form) Varies with type of exercise (running vs. swimming)

Figure 5.7

Energy Expenditure: Energy Cost of Various Activities Varies with type and intensity of activity Calculated from VO2, expressed in kilocalories/minute Values ignore anaerobic aspects, EPOC Daily expenditures depend on Activity level (largest influence) Inherent body factors (age, sex, size, weight, FFM)

Table 5.2

Energy Expenditure: Successful Endurance Athletes 1. High VO2max 2. High lactate threshold (as % VO2max) 3. High economy of effort 4. High percentage of type I muscle fibers

Fatigue and Its Causes Fatigue: two definitions Reversible by rest Decrements in muscular performance with continued effort, accompanied by sensations of tiredness Inability to maintain required power output to continue muscular work at given intensity Reversible by rest

Fatigue and Its Causes Complex phenomenon Type, intensity of exercise Muscle fiber type Training status, diet Four major causes (synergistic?) Inadequate energy delivery/metabolism Accumulation of metabolic by-products Failure of muscle contractile mechanism Altered neural control of muscle contraction

Fatigue and Its Causes: Energy Systems—PCr Depletion PCr depletion coincides with fatigue PCr used for short-term, high-intensity effort PCr depletes more quickly than total ATP Pi accumulation may be potential cause Pacing helps defer PCr depletion

Fatigue and Its Causes: Energy Systems—Glycogen Depletion Glycogen reserves limited and deplete quickly Depletion correlated with fatigue Related to total glycogen depletion Unrelated to rate of glycogen depletion Depletes more quickly with high intensity Depletes more quickly during first few minutes of exercise versus later stages

Figure 5.8

Fatigue and Its Causes: Energy Systems—Glycogen Depletion Fiber type and recruitment patterns Fibers recruited first or most frequently deplete fastest Type I fibers depleted after moderate endurance exercise Recruitment depends on exercise intensity Type I fibers recruit first (light/moderate intensity) Type IIa fibers recruit next (moderate/high intensity) Type IIx fibers recruit last (maximal intensity)

Figure 5.9

Fatigue and Its Causes: Energy Systems—Glycogen Depletion Depletion in different muscle groups Activity-specific muscles deplete fastest Recruited earliest and longest for given task Depletion and blood glucose Muscle glycogen insufficient for prolonged exercise Liver glycogen  glucose into blood As muscle glycogen , liver glycogenolysis  Muscle glycogen depletion + hypoglycemia = fatigue

Figure 5.10

Fatigue and Its Causes: Energy Systems—Glycogen Depletion Certain rate of muscle glycogenolysis required to maintain NADH production in Krebs cycle Electron transport chain activity No glycogen = inhibited substrate oxidation With glycogen depletion, FFA metabolism  But FFA oxidation too slow, may be unable to supply sufficient ATP for given intensity

Fatigue and Its Causes: Metabolic By-Products Pi: From rapid breakdown of PCr, ATP Heat: Retained by body, core temperature  Lactic acid: Product of anaerobic glycolysis H+ Lactic acid  lactate + H+

Fatigue and Its Causes: Metabolic By-Products Heat alters metabolic rate –  Rate of carbohydrate utilization Hastens glycogen depletion High muscle temperature may impair muscle function Time to fatigue changes with ambient temperature 11°C: time to exhaustion longest 31°C: time to exhaustion shortest Muscle precooling prolongs exercise

Figure 5.11

Fatigue and Its Causes: Metabolic By-Products Lactic acid accumulates during brief, high-intensity exercise If not cleared immediately, converts to lactate + H+ H+ accumulation causes  muscle pH (acidosis) Buffers help muscle pH but not enough Buffers minimize drop in pH (7.1 to 6.5, not to 1.5) Cells therefore survive but don’t function well pH <6.9 inhibits glycolytic enzymes, ATP synthesis pH = 6.4 prevents further glycogen breakdown

Figure 5.12

Fatigue and Its Causes: Lactic Acid Not All Bad May be beneficial during exercise Accumulation can bring on fatigue But if production = clearance, not fatiguing Serves as source of fuel Directly oxidized by type I fiber mitochondria Shuttled from type II fibers to type I for oxidation Converted to glucose via gluconeogenesis (liver)

Fatigue and Its Causes: Neural Transmission Failure may occur at neuromuscular junction, preventing muscle activation Possible causes –  ACh synthesis and release Altered ACh breakdown in synapse Increase in muscle fiber stimulus threshold Altered muscle resting membrane potential Fatigue may inhibit Ca2+ release from SR

Fatigue and Its Causes: Central Nervous System CNS undoubtedly plays role in fatigue but not fully understood yet Fiber recruitment has conscious aspect Stress of exhaustive exercise may be too much Subconscious or conscious unwillingness to endure more pain Discomfort of fatigue = warning sign Elite athletes learn proper pacing, tolerate fatigue